Methods of growing crystals and making electrical translators



A nl 9, 1957 E. N. CLARKE 2,738,298

METHODS OF GRQWING CRYSTALS AND ELECTRICAL TRANSLATQRS Filed Nov. 2,1951 4 E ti m lslwl Z8 5 .& y g s 3'40 E INVENTOR' 50 EDWARD N.CLARKE BYw 5 4 3 -z 0 ma ATTORNEY www- United States Patent METHODS OF GROWINGCRYSTALS AND MAKING ELECTRECAL TRANSLATORS Edward N. Clarke, .Levittown,N. Y., assignor to Sylvania Electric Products 1126., a corporation ofMassachusetts Application November 2, 1951, Serial No. 254,478

2 Claims. ((31. 148-15) The present invention relates to semiconductortrans lators, particularly to electrical devices of this class used forrectification and for amplification purposes.

The possibility of producing amplification and related translatingfunctions by means of a thin layer of semiconductor of one typesandwiched between two bodies of the opposite type of semiconductor isknown to the art. These may be termed area-junction translators. Theobject of the present invention is to provide a novel, effective methodfor forming in integral assembly a single body of a semiconductor havinga thin layer of one type of semiconductor interposed between theopposite types of semiconductor. The thin interposed layer may be eitherN-type or P-type, and correspondingly the end portions of the elementwill be P-type or N-type, respec tively. A further object is to producearea-junction translators by a novel, effective method.

As will be seen from the illustrative disclosure de scribed below, inwhich the method is practiced using germanium containing appropriateimpurity constituents, it is possible to produce a thin layer of onetype of semiconductor when growing a crystal from a melt where the bulkof the crystal produced is of the other type merely by adjusting thepulling rate and the temperature in such fashion that the crystal beinggrown breaks away from the melt. This provides a barrier useful inmaking area junction rectifiers. The pulled crystal thus broken canagain be dipped into the molten material for renewed growth. Where themolten material is such as to produce an N-type crystal, the break andrenewed growth results in a P-type layer of extreme thinness. Where themelt is of such character as normally results in production of a P-typecrystal, the layer produced as a result of breaking the smoothcrystal-growing process followed by renewed dipping and further smoothgrowth is an N-type layer. Generally, and viewed apart from thedesignations of the particular semiconductor types, the invention isconcerned with the formation of an electrical barrier and with formingmultiple barriers close to each other in a unitary semiconductor body,and with the manufacture of semiconductor translators with such bodies.Further, the invention is concerned with introducing distinctiveinhomogeneities in a layer of a crystal grown out of a melt.

The thickness and electrical properties of the layer are influencedsomewhat by numerous factors which may be varied by the practitioner ofthe invention, including the time that the broken crystal is allowed toremain suspended above the melt, the speed of withdrawing the redippedend of the crystal, the extents to which the broken crystal is separatedfrom the melt and re-dipped into the melt, and by such additionalfactors as temperature, temperature gradient, the extent and types ofimpurities in the melt, whether a gas is enclosed or the furnace isevacuated, and the time between the break and the redipping of thecrystal for renewed growth of the crystal.

' The'na'ture of the invention and its further aspects of novelty willbe apparent from the following detailed description of an illustrativeembodiment thereof shown in the accompanying drawings.

in the drawing, Fig. l is the schematic view of a furnace, inlongitudinal cross-section, suitable for carrying out the novel process.Figs. 2, 3., and 4 are somewhat on (.3 views which illustrate thesequence of steps in the 11 ustrative embodiment of the invention. Fig.5 is an illustrative translator incorporating the distinctive layer, andFig. 6 is a family of performance curves of the device in Fig. 5.

Referring to Fig. 1 there is shown a hard glass container to enclosingthe crucible 12, which crucible is heated by resistance wire 14 havingleads 16 extending outside the chamber 10. The resistance wire is woundwith such distribution, and is properly subdivided, to give control overthe temperature gradient in the melt. Ceramic walls 18 support theheating wire and prevent excess transfer of heat to the exterior. Rod 20extends to a gland or vacuum seal 22 and can be manipulated for rotationand for axial travel whereby it is possible to lower and elevate a seed24 as of crystalline germanium into the melt 23 of suitably dopedgermanium. A crystal 30 is shown being grown by gradually raising rod 20after seed 2% has been immersed in the molten bath 28. The temperatureof the molten bath is maintained critically, as a matter of judgment, inrelation to the speed of with drawal of rod ill. The crucible is exposedto a helium atmosphere in growing both N-type and P-type germaniumcrystals. The atmosphere is continuously changed by flowing pure, coolgas into and out of chamber 10, through tubes 32.

in a typical process of growing a crystal 30, germanium is loaded inpowder form or in pieces into crucible 12, together with any dopingconstituent that may be desired, and the furnace is heated by energizingresistance wire 14 after the chamber has been flushed with helium. Afterthe material that is to form the semiconductor has become molten andstabilized at a desired temperature the seed 24 of approximately thesame material but in solid crystalline form (being secured at the bottomof rod 2t?) is brought into contact with the surface of the melt andthereafter is slowly withdrawn vertically. The molten germanium incontact with the seed (and there after with the growing crystal)gradually solidifies and adds to the crystal as it is raised. Suitablythe rate of withdrawal may be 6 inches per hour and the germanium may benear its melting point, slightly below the melting point so as to besupercooled at the solid-liquid interface.

At the outset the germanium used is of a high order of purity and it maycontain in mixture or in alloy form a small amount of dopingconstituents and predominantly of a doping constituent such as to forman N-type or a P-type semiconductor. It appears probable that slightamounts of both donor and acceptor impurities are inherently present inthe germanium used (although below spectroscopically detected amounts)apart from the doping constituent deliberately added.

As shown in Figs. 2, 3, and 4 a barrier of the opposite type ofsemiconductor can be formed in a crystal of a given type ofsemiconductor by the abrupt withdrawal of the growing crystal from thesurface of the melt and, after a pause, reimmersion of that growingcrystal. The crystal should remain suspended until the molten germaniumat its bottom face has become solidified. Purely as a matter ofillustration the germanium when withdrawn should be raised from thesurface of the melt by no more than and, for a growing crystal of /8diameter in a flowing helium atmosphere, no more than 5 seconds arerequired for solidification of the molten germanium at the bottom of thegrowing crystal to be completed, after which the crystal can again bebrought into contact with the germanium melt and further growing of thecrystal can be resumed.

In Fig. 3 the surface of the growing crystal 30 that has just beenwithdrawn from the melt 28 is designated lrtla. This is molten initiallyand solidifies rather rapidly in the ga atmosphere of the furnace. Zone30b in Fig. 4

represents the physical appearance of the crystal that has been grown asin Fig. 2, broken as in Fig. 3, and where renewed crystal growth hastaken place. The transverse dimension of the approximately cylindricalcrystal 3% may be of the order of a as" or it can be much smaller orlarger; and the region where the irregular appearance 30b is noticed maybe about measured along the crystal axis. Irrespective of the axialextent of this break in the crystal growth, the layer of a contrastingtype of semiconductor that is formed in the crystal produced may be nomore than a thousandth of an inch or possibly as high as fivethousandths of an inch, depending upon all conditions connected with themelt, the interruption of crystal growth, and the re-dipping. Thejunction of the grown crystal 30 with the seed 24 shows a continuity ofthe crystalline structure as evidenced by visual examination afteretching the surface with a ferric chloride hydrochloric acid etch; andthis continuous crystalline structure is evident despite theirregularity of contour at zone 30b, in most junctions formed.

It is possible to so cool the crystal after raising it from the melt,and to dip it and promptly start raising it in resumed crystal growing,that the germanium of the melt merely adheres to the dipped crystal. Itis also possible to immerse the end of the re-dipped crystal too deepand-for too long a time, so as to obliterate the junction entirely.However, it is possible by adjusting the technique properly to obtain anintegral continuity of crystal growth with a distinctive barrier layerof a contrasting semiconductor type in a large crystal of the oppositesemiconductor type. This is the desired result. The technique ofinterrupting the growth of the crystal by removing it to a spacedposition above the melt and again dipping it can be repeated so as toprovide a sequence of thin contrasting layers extending all the wayacross the crystal being grown at spaced positions along its axis.

In forming semiconductor translators a crystal such as that shown inFig. 4 is sub-divided by a transverse cut above and below zone 3% andthereafter by perpendicular families of cuts through the slice soformed. Before sub-dividing the slice, its opposite faces areadvantageously electroplated as with copper to provide ohmic terminalconnections. This provides individual semiconductor translator elementswith two ohmic contacts 40 and 42 at the ends of the element and a thirdohmic contact 44 is made to the interposed contrasting layer. It isdesirable to polish and etch the lateral surfaces for removing foreignmaterial and deformed mechanically worked parts of the crystal in thebarrier region, and to facilitate establishing contact 44 properly.Portions 30' (Fig. may be of N-type and in that event portion 30b is ofP-type germanium; and on the other hand, if the melt is predominantly ofsuch character as to produce P-type portions 30' then portion 39b is ofN-type semiconductor.

The portion 30b can be located simply by probing. One effective way ofdoing this is to rest the etched crystal against a metal base extendingfrom terminal 40 to terminal 42 and to connect one of the terminals 44or 42 to a galvanometer circuit with the opposite terminal of thegalvanometer circuit constituted of a soldering iron suitably heated,having a sharp contact point at its end. At the instant that the probetouches the semiconductor on a lateral face the galvanometer will kick.The direction that it kicks, whether positive or negative, is anindication of the semiconductor in that immediate region, regardless ofhow thick or thin. Other probing t ch iques are well known to thoseskilled in the art.

Another way of locating the layer is by selective etching with an aceticacid, hydrofluoric acid, nitric acid etching solution containingbromine.

After the portion 30b has been located, contact 44 is established thatshould ideally be ohmic. Phosphor bronze is desirable for this purpose,the connection of the phosphor bronze wire to the layer becomingefitective after brief operation and/or electrical pulsing if it is notactually suitable at the outset. The dimensions of the abutting end ofcontact 44 against layer 39b are fixed by the thickness of the layerformed and the contact 44. will naturally be of a very sharp-ended wirewhere the layer is no more than a thousandth of an inch. Severalcontacts may be made about the circumference of layer 3% which contactsshould be interconnected, but for most purposes a single connection issuitable. Any suitable mechanical support may be used to hold contact 44in position.

The unit in Fig. 5 may be employed in a variety of circuits but withinput and bias connections between terminals 42 and 44, static output(collector) characteristics of the circuit between terminals 40 and 44may be obtained as shown in Fig. 6. Ie is the input-circuit current.

I have no theory with which I am thoroughly satisfied as to themechanism of accomplishing the foregoing effect, and the following isoffered by way of explanation only. It is Well known that when a longmelt of germanium is gradually allowed to solidify progressively fromone end of the melt to the other the ingot formed very often hasportions of semiconductor of different characteristics at its ends. Thisis said to be due to a migration of impurities from the portion beingsolidified to the portion still molten. As the germanium that is moltenat region 30a gradually solidifies, it may be that certain impurities inthat limited molten volume migrate towards the surface not yetsolidified, that is, the surface directly opposite melt 28, and exposedto the heat of the melt. The solidification of the limited quantityresults in the formation of a thin layer of a contrasting type ofsemiconductor. Such layer is effective as an area rectifier or aphotodetector when properly formed terminals are applied, without anyre-dipping. When the bulk is N-type, the skin formed is P-type. Afterredipping, the layer formed is fused to the further mass of crystal thatis grown thereafter. The crystalline structure that is first formed inthe region 30 develops with a certain crystalline orientaiton relativeto the solidliquid interface; and this same crystal orientaiton isrealized in renewed growth after dipping, judging from the appearance ofan etched grown crystal that has been broken and re-dipped.

The foregoing disclosure of methods for producing a thin layer ofsemiconductor in or on a larger body of a different form ofsemiconductor, and the conclusion of semiconductor translator using suchcomposite semiconductor, will be recognized by those skilled in the artas subject to a latitude of variations. Those skilled with the art ofsemiconductors and with the art of growing crystals will recognize avariety of application of the various aspects of the inventionrepresented in this disclosure; and accordingly the appended claimsshould be allowed that broad scope of interpretation consistent with thespirit and scope of the invention.

What is claimed is:

1. The method of making an electrical translator, including the steps ofgrowing a crystal out of a germanium melt containing traces ofconductivity-type determining impurities, during which operationimpurities present in the melt tend to separate from the germanium thatis in the process of solidifying on the growing end of the crystal, suchimpurities thereby separating preferentially between the growing crystaland the melt, removing the growing crystal from the melt with a moltenlayer of germanium on its end containing entrapped impurities, causingsuch molten layer to become solidified thereby to produce a layer ofgermanium of conductivity type opposite to that of said crystal, andapplying electrical terminals to the solidified layer and to anotherportion of the grown crystal.

2. The method of making an electrical translator, including the steps ofgrowing a crystal out of a germanium melt containing conductivity-typedetermining impurities, during which operation impurities present in themelt tend to separate from the germanium that is in the process ofsolidifying on the growing end of the crystal, such impurities therebyremaining preponderantly in the melt, removing the growing crystal fromthe melt with a molten layer of germanium on its end containingentrapped impurities, causing such molten layer to become solidifiedthereby to produce a layer of germanium of conductivity type opposite tothat of said crystal, engaging such solidified layer with the melt in away to enable resumed growth of the crystal and resuming growing of thecrystal,

and thereafter applying terminals to the solidified layer and to theportions of the crystal on opposite sides thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,402,582 Scait June 25, 1946 2,402,661 Ohl June 25, 1946 2,565,338Amico .Aug. 21, 1951 2,576,267 Scatf et a1 Nov. 27, 1951 2,583,008 OlsenJan. 22, 1952 2,623,102 Shockley Dec. 23, 1952 2,623,103 Kircher Dec.23, 1952 2,631,356 Sparks et a1 Mar. 17, 1953 OTHER REFERENCESPreparation of Metal Single Crystals," by A. N. Holden. ASM 1949Rreprint No. 35, also published in Transactions, American Society forMetals, vol. 42 (1950).

1. THE METHOD OF MAKING AN ELECTRICAL TRANSLATOR, IN CLUDING THE STEPSOF GROWING A CRYSTAL OUT OF A GERMANIUM MELT CONTAINING TRACES OCONDUCTIVITY-TYPE DETERMINING IMPURITIES, DURING WHICH OPERATIONIMPURITIES PRESENT IN THE MELT TEND TO SEPARATE FROM THE GERMANIUM THATIS IN THE PROCESS OF SOLIDIFYING ON THE GROWING END OF THE CRYSTAL, SUCHIMPURITIES THEREBY SEPARATING PREFERENTIALLY BETWEEN THE GROWING CRYSTALAND THE MELT, REMOVING THE GROWING CRYSTAL FROM THE MELT WITH A MOLTENLAYER OF GERMANIUM ON ITS END CONTAINING ENTRAPPED IMPURITIES, CAUSINGSUCH MOLTEN LAYER TO BECOME SOLIDIFIED THEREBY TO PRODUCE A LAYER OFERMANIUM OF CONDUCTIVITY TYPE OPPOSITE TO THAT OF SAID CRYSTAL, ANDAPPLYING ELECTRICAL TERMINALS TO THE SOLIDIFIED LAYER AND TO ANOTHERPORTION OF THE GROWN CRYSTAL.